Waveguide components



Ap 1960 M. GUMBRELL 2,934,725

WAVEGUIDE COMPONENTS Filed Oct. 28. 1957 INVENTOR Win 161. 6: 4 aRGLLFITTORNE Y5 United States Patent WAVEGUIDE COMPONENTS ApplicationOctober 28, 1957, Serial No. 692,890

Claims priority, application Great Britain October 26, 1956 9 Claims.(Cl. 333-98) This invention relates to waveguide components.

'If a longitudinal slot is provided in the wall of a waveguide throughwhich electromagnetic energy is being propagated, the direction of theelectric field within' the waveguide in the vicinity of the slot beingacross the slot, then it can be shown that some of that energy isradiated from the slot .and thereby lost unless steps are taken toprevent such radiation.

One object of the present invention is to provide a waveguide componentin which there is a} longitudinal slot in the wall of a waveguide butwhich is arranged so as to prevent any appreciable energy being lostthrough the slot. a 1

Another object is to provide such a slotted waveguide in which the lossof energy from the waveguide is prevented over a wide range ofwavelengths of the electromagnetic wave being propagated through theWave guide.

Various waveguide components have been proposed in which a body offerrite material is disposed within a Waveguide of circularcross-section, the ferrite body being subjected to a magnetic field in adirection parallel to the longitudinal axis of the waveguide. Some suchcomponents, for example, attenuators and gyrators, depend for theiroperation on the Faraday rotation of the plane of polarisation of theelectromagnetic wave within the waveguide. The said magnetic field isusually produced by a coil embracing the waveguide in the region of theferrite body and the transmission properties of the waveguide componentmay be changed by varying the current carried by the coil. If thewaveguide component is required to present two different conditions oftransmission periodically and alternately, this may be done by supplyinga suitable electric signal to the coil. In some applications, forexample in .a radar system for the purpose of switching betweentransmitting and receiving conditions of the system, it is desirable tobe able to change between these two conditions rapidly and frequentlyand the current supplied to the coil then contains a component ofrelatively high frequency. Since, however, the wall of the circularwaveguide .efiectively constitutes a short-circuited turn in which poweris dissipated, the adverse effect of the short-circuited turn increasingwith frequency, steps have to'be taken to reduce the power that wouldotherwise be required to energise the coil. One suggested way of doingthis is to reduce the thickness of the waveguide wall so as to increasethe resistance thereof at the switching frequency but there is a limitto the amount the thickness of the waveguide wall can be reducedwithout'producing leakage of the electromagnetic energy which ispropagated through the waveguide.

A further object of the present invention is to ro.- vide a waveguidecomponent in which the difliculty dis-. cussed in the last paragraph isovercome.

According to the present invention, a waveguide component comprises awaveguide having a longitudinal slot in a wall thereof and a memberlying outside the wave guide and adjacent to the said slot so that thesaid member and the waveguide together form a transmission line which isarranged to be excited in an evanescent mode when an electromagneticwave is being propagated along the waveguide so that no appreciableportion of the electromagnetic energy is lost through the slot.

According to 'a' feature of the present invention, a waveguide componentcomprises a waveguide which is of circular cross-section, a body (whichmay consist of a pluralityof separate portions) of ferrite material.dis.- posed within the waveguide, a coil or coils surrounding thewaveguide tor the purpose of subjecting the ferrite body to amagneticfield at least a componentlof which is in a direction parallel to thelongitudinal axis of the waveguide, the waveguide wall having alongitudinal slot therein which is approximately co-terminous with thefer.- rite body and the waveguide being arranged so that it does notconstitute a short-circuited turn embracing the said body, and a memberwhich lies outside the waveguide and adjacent to the said slot and whichtogether with the waveguide forms a transmission line which is arrangedto be excited in an evanescent mode when an electromagnetic wave isbeing propagated along the waveguide so that no appreciable portion ofthe electromagnetic energy is lost through the slot.

The longitudinal slot of a waveguide component in accordance with thepresent invention may be parallel to the longitudinal axis of theWaveguide but it is to be understood that this feature is not necessary,the expression longitudinal slotfmerely implying that the-direction ofthe slot has a component parallel to the longitudinal axis. v

The manner of operation of a waveguide component in accordance with thepresent invention will now be discussed with reference to Figurel of theaccompanying diagrammatic drawing which shows a cross-sectionalview of awaveguide which is assumed to be of infinite length. The waveguide,which has the reference 1 in the drawing, is of uniform circularcross-section; along its length and is filled with a homogeneouslossless material, therefractive index of this material toelectromagnetic waves being N A slot 2 is provided in the wall 3 of thiswaveguide 1 in a direction parallel to the longitudinal axis of: thewaveguide, this slot also being assumed to be of infinite length, whileoutside the waveguide 1 and adjacent to the slot 2 along the whole ofits length there is provided an arcuate metal member '4,. This metalmember 4; lies equally on either side of the slot 2 and the adjacentsurfaces Sand 6 of the waveguide wa1l3 and the member 4 are spaced auniform distanceapart, ,The material in the space 7 between the member 4and the waveguide Wall 3 has a refractiveindex N If now anelectromagnetic Wave which is circularly polarised in the dominant Hmode 'is propagated along the waveguide 1, it is convenient to considersuch a wave as being made up of two plane-polarised waves which are intime and space quadrature, one of these component waves being planepolarised in the plane of the slot 2 that is to say parallel to thearrow 8, while the other-is Patented Apr. 26, 1960 polarised in a planeperpendicular to the plane of the slot, that is to say parallel to thearrow 9. Of these two components, only the one that is polarised in theplane parallel to the arrow 9 is coupled through the slot 2 to theparallel plate transmission line formed by the member 4 and thewaveguide wall 3. The amplitude of this coupling varies sinusoidallyalong the length of the slot and the pattern of excitation in thetransmission line moves in the direction parallel to the longitudinalaxis of the waveguide (that is to say at right angles to the plane ofFigure 1') at the speed of propagation of the wave in the waveguide 1.

The traveling pattern of excitation in the transmission line formed bythe member 4 and the waveguide wall 3 can be considered as being theresultant of two standing wave patterns which are of equal amplitude andwhich are spaced a quarter of a wavelength apart and in time quadrature.Since, however, the factors effecting the propagation in the saidtransmission line resulting from each of these standing wave patterns isthe same, it is only necessary to consider the propagation resultingfrom a single standing excitation pattern. If it is assumed that thespacing of the member 4 and the waveguide wall 3 is small enough toensure that modes of excitation involving components of electric fieldin directions parallel to the conductors of the said transmission linecannot be propagated, it follows that the only possible mode ofpropagation excited by this particular standing wave pattern is one inwhich the electric lines extend between the two conductors so as to benormal to the surfaces 5 and 6 while the magnetic lines are orthogonalto the electric lines and form loops which are disposed in a successionof rows, each of these rows being parallel to the 'slot 2 in a directionperpendicular to the plane of Figure l. p

The transmission line is arranged (as hereinafter described) so that ifit were to propagate a Wave in the mode discussed in the last paragraph,the wavelength of that wave in the transmission line would be greaterthan the wavelength of the exciting wave in the waveguide 1. This is, ofcourse, not possible with the result that the transmission line cannotbe excited in the mode discussed to propagate a wave and it is thereforeexcited in an evanescent mode. Thus any wave coupled into the trans-1nission line through the slot 2 is rapidly attenuated and does notreach the open ends 10 and 11 of the line. qccordingly substantially noenergy is lost through the "s ot 2.

The transmission line formed by the member 4 and waveguide wall 3 isexcited in an evanescent mode, as aforesaid, if the'following inequalityis satisfied.

2 g o where a is the wavelength measured longitudinally in the waveguideof the electromagnetic wave being propagated through the'wavelength 1and A is the free space wavelength of that wave. In other words A mustbe less than the natural wavelength where h is the cut-ofi wavelengthfor the waveguide 1 the H11 mode.

Although the above discussion is based on the assumption that theWaveguide 1 is filled with a homogeneous material of refractive index Nthe inequality (1) still holds if the waveguide 1 is partially filledwith a body,

such as a ferrite rod which is shown by a broken outline inclined to thelongitudinal axis of the Waveguide.

shaped member 17 of polystyrene.

4 12, having one refractive index while the rest of the waveguide isfilled with material having a different refractive index.

Furthermore, although the above discussion has assumed the waveguide 1to be passing an electromagnetic wave that is circularly polarised, itis equally applicable to plane-polarised wave, which may have acontinuously rotating plane of polarisation along its direction ofpropagation, since a plane-polarised wave can be considered as beingformed by two circularly-polarised Waves of equal amplitude and oppositesenses of rotation.

The above discussion is concerned with a waveguide having a longitudinalslot in the waveguide wall that is parallel to the longitudinal axis ofthe waveguide but this limitation as to the direction of the slot is nota necessary feature of the invention as previously mentioned. In fact,if the waveguide is required to pass a circularlypolarised wave, thelongitudinal slot may preferably be The reason for this is that if theslot is in the form of arighthanded helix which has a pitch of 25radians per unit length while the wave itself has a left-handpolarisation (using the terminology of section 3.12 of Microwave AntennaTheory and Design, volume 12 of the Massachusetts Institute 'ofTechnology Radiation Laboratories Series, edited by S. Silver), theinequality (1) above is replaced by panying drawing which shows anisometric view of the component. For the purpose of showing theconstruction of the component, it is shown in Figure 2 partially inexploded form and partially sectioned with the outer layers removed.

Referring now to'Figure 2, the component comprises a waveguide 13 ofcircular cross-section in which is supported a ferrite'rod 14 which ismade up of a plurality of likemain portions 14a and two like endportions 14b and the rest of the waveguide is filled with polystyrene.In

fact the waveguide 13 is formed by silvering the outer surface of a tube16 of polystyrene by a suitable chemical process and then electroplatingthe silver layer so as to build up the waveguide Wall 15 with eithersilver or copper to I a thickness of approximately five thousandths 'ofan inch.

The portions 14a of the ferrite rod are of uniform diameter which isslightly less than the inner diameter of the polystyrene tube 16 whilethe end portions 14b are tapered so as, in known manner, to minimise theimpedance discontinuities presented by the rod 14. During manufacture,the rod 14 is pushed into position within the tube 16 and each end ofthis tube is plugged with a When so assembled, the-end 18 of the member17 is fiush with the end 19 of the polystyrene tube 16. A slot 20 whichlies parallel to the longitudinal axis 21 of the waveguide is providedin the waveguide wall, this slot being approximately coterminous withthe ferrite rod 14. This slot is formed during manufacture of thewaveguide by sticking a filament or thread (not shown) along thepolystyrene tube "16 during the silvering process and then stripping offthis filament-or thread prior to the electroplating. In similar mannertwo circumferential slots (of which only the slot 22'is shown in Figure2) are provided in the waveguide wall 15 at each end of the longitudinalslot 21, the two circumferential slots being connected to thelongitudinal slot so that the portion of the waveguide wall 15embracthe-ferrite rod 14 does not; constitute a short-circuited i Thecircumferential "slot 22,,fo1, example, lies'in the plane containing theedge 23. i A rectangular metal plate 24 which is bent to an arcuateshape is mounted over the longitudinal slot 20. The spacing between theadjacent surfaces of the plate 24 and the waveguide wall 15 is uniformand the dielectric inthe transmission line formed thereby is mainly air.This plate 24 is in fact spaced from the waveguide wall 15 by thinstrips 25 of a suitable solid dielectric material such as polyethylene,these strips partially embracing the waveguide 13. Alternatively stripsof polyethylene may, for this purpose, form loops completely embracingthe'waveguide 13 or be wound round the waveguide 13 in helical manner.The plate 24 is held in position against these strips 25 by tapes 26which are wound round the plate 24 and the waveguide 13 together.

The magnetic field in which the ferrite rod 14 lies during use of thecomponent being described, is provided by a coil 27. This coil 27 iswound between cheeks 28 over the assembly of the waveguide 13 and theplate 24. The coil 27 may be wound on a former (not shown) which isplaced over the assembly.

Metal coupling flanges 29 are provided at each end of the waveguide 13,these flanges being electrically connected to the waveguide wall 15.

Typical dimensions of the waveguide component dercrfibed above and theparameters of the materials are as o ows:

The length of the rod 14 and thus of the slot 20 and plate 24, is not ofcourse critical, as far as the present invention is concerned, and ischosen to give the component the desired electrical characteristics.

The waveguide component may be a gyrator and in that case the coil 27 isexcited, during use, so that a wave passing through the waveguide 13 inone direction is subjected to a phase shift of 180 while a wave passingin the opposite direction does not undergo any phase shift. The phaseshift to which waves passing through the waveguide 13 in the twodirections are subjected may be interchanged by reversing the flow ofenergizing current through the coil 27. For this purpose switching means(not shown) are connected in circuit with the coil 27.

It will be appreciated that in the waveguide component described above,the wavelength A is a function of the amplitude of the magnetic fieldprovided by the coil 27 and the inequality (1) must therefore besatistfied forv the largest value of this wavelength over the desiredrange of amplitude of the magnetic field. Since however the wavelengthchanges in this manner, it will be appreciated that it would not bepossible to prevent radiation from the longitudinal slot 20 in thewaveguide wall 15 by means of a conventional resonant choke arrangement.Such radiation is, however, prevented by providing the transmission linewhich is formed by the member 24 and the waveguide wall 15 and which isexcited in an evanescent mode in accordance with the present invention.

If the component is required for use in a waveguide system which is madeup mainly of waveguides of rectangular cross-section, lengths ofwaveguide for convertwe e 6 ing' from circular to rectangularcross-section may be connected one to each end of the waveguide 13.

For symmetry, it may be convenient in some circum stances to providemore than one longitudinal slot in the waveguide wall 15 and, forexample, four equally spaced slots may be provided, each of these slotshaving its associated transmissionline which is arranged to excited inan evanescent mode. I

I claim:

1. A waveguide component comprising a waveguide which has a longitudinalslot in a wall thereof and which is filled with dielectric material, amember lying outside the waveguide and adjacent to the 'said slot, anddielectric material which is disposed between the waveguide wall andsaid member and which has a refractive index less than that of thedielectric material filling the waveguide so that the said member andthe waveguide together form a transmission line which is arranged to beexcited in an evanescent mode when an electromagnetic wave is beingpropagated along the waveguide so that no appreciable portion of theelectromagnetic energy is lost through the slot.

2. A waveguide component according to claim 1 wherein the said waveguideis of circular cross-section.

3. A waveguide component comprising a waveguide which is of circularcross-section, a body of ferrite material disposed within the waveguide,first dielectric material filling the space between the ferrite body andthe waveguide wall, a coil or coils surrounding the waveguide for thepurpose of subjecting the ferrite body to a magnetic field at least acomponent of which is in a direction parallel to the longitudinal axisof the waveguide, the waveguide wall having a longitudinal slot thereinwhich is approximately co-terminous with the ferrite body and thewaveguide being arranged so that it does not constitute ashort-circuited turn embracing the v said body, a member which liesoutside the waveguide and adjacent to the said slot, and seconddielectric material filling the space between the waveguide wall andsaid member, the refractive index of the second dielectric materialbeing less than the effective refractive index of the waveguide fillingconstituted by the first dielectric material and the ferrite body sothat the member and the waveguide together form a transmission linewhich is arranged to be excited in an evanescent mode when anelectromagnetic wave is being propagated along the waveguide so that noappreciable portion of the electromagnetic energy is lost through theslot.

4. A waveguide component according to claim 3 wherein at each end of thesaid longitudinal slot there is a circumferential slot in the waveguidewall so that the portion of the waveguide wall that embraces the saidbody does not constitute a short-circuited turn.

5. A waveguide component according to claim 1 wherein the saidlongitudinal slot is parallel to the longitudinal axis of the waveguide.

6. A waveguide component according to claim 1 wherein the saidlongitudinal slot lies on a helix.

7. A waveguide component according to claim 1 wherein the saidtransmission line lies on either side of the said slot along the lengthof the slot.

8. A waveguide component according to claim 1 wherein there is uniformspacing between the said member and the waveguide wall.

9. A waveguide component comprising a waveguide formed by a tube whichis of circular cross-section and which has a wall of uniform thicknesswith an elongated opening in the wall extending longitudinally of thetube, an arcuate member having an uninterrupted electrically conductingsurface, and means to mount the arcuate member outside the tube andadjacent to the opening so that along the length of the opening and oneither side thereof the said wall and said surface of the arcuate membertogether form a transmission line and at the frequency of anelectromagnetic wave being propagated along the wavegnide the naturalwavelength of a wave in the transmission line is greater than that inthe waveguide.

References Cited in the file of this patent UNITED STATES PATENTSSchelkunoff Apr. 25, I939 Llewellyn Nov. 11, 1941 Llewellyn Feb. 26,1946 Carlson July 2, 1946 Kallmann Feb. 8, 1949 Collard June19, 1951FOREIGN PATENTS v France Jan. 7, 1957 OTHER REFERENCES 19 5 Montgomery:Technique of Microwave Measurements Radiation Laboratory Series,McGraw-Hill 1947, pages 489, 491 and 493. I I

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